Metal carbides, such as tungsten carbide, are used in making a variety of hard and/or high temperature materials. Metal carbides are made conventionally by blending carbon with metal powder and carburizing the metal at elevated temperatures. This procedure produces a metal carbide powder whose properties. (high hardness, high melting) are not amenable to permitting the material to be readily manipulated into other forms such as coatings, fibers, or shaped objects.
Pat. application Ser. No. 727,524, incorporated herein by reference, addresses the problem of how to readily produce metal carbides, nitrides, and the like in the form of coatings, fibers or shaped objects. The application generally describes a procedure for pyrolyzing tractable organometallic precursors in which organic ligands are bonded to the metal. The tractability of the precursor enables it to be placed in the desired form before it is pyrolyzed.
U.S. Pat. application Ser. No. 900,592, derived from the '524 application and also incorporated by reference herein, involves the use of a particular class of organometallic precursors, polynuclear metal amides, that contain two or more metal atoms. These compounds were found unexpectedly to be metal carbide precursors. In this regard, prior workers reported in J. Electrochem. Soc., (1975) 122:1545-1549 that mononuclear metal dialkylamides which contain one metal atom thermally decomposed in the gas phase to produce metal nitrides. It was expected, therefore, that the polynuclear metal amide precursor would yield metal nitrides rather than metal carbides. It was further found that metal carbides in which the carbon is in excess and is bound covalently to the metal may be produced by pyrolyzing certain metal amides containing two or more metal atoms. Such metal carbides are believed to be novel. Like the precursors of the '524 application, the transition metal amide precursors of the '592 application may be readily converted to coatings or shaped articles of metal carbide using low temperature pyrolysis.
The art directly relating to metal carbides in which there is excess carbon is exemplified by U.S. Pat. No. 3,865,745 and Refractory Hard Metals: Borides, Carbides, Nitrides and Silicides, Schwarzkopf, P., et al, MacMillen and Co., NY, (1953). The patent describes the preparation of metal carbide microspheres by heating in the presence of steam microspheres of ion exchange resins that have been pretreated with metal ion solution. The patent indicates that the carbon in the microspheres can be in molar excess depending upon the heating conditions. It is believed that the carbon in this product is not present in the form of a covalently bound species, but is instead present in the form of excess carbon (graphite) in solid solution. Also, in view of the presence of steam in the process, the product is contaminated by oxycarbide to greater or lesser extent depending on the heating conditions.
The present application relates to certain aspects of the precursors, processes and products described in patent applications Ser. Nos. 727,524 and 900,592, in particular: (1) a process for preparing metal nitrides, borides, silicides, sulfides and phosphides via low-temperature pyrolysis of selected precursors; and (2) that process the process of the '524 application; and (3) transition metal carbides produced by pyrolysis of polynuclear transition metal amides according to the process of the '392 application.